In recent years, inductive charging technology has become a leading candidate to eliminate power cables. Inductive power systems and other contactless power systems typically use one or more transmitters to send power to one or more receivers. Electronic devices with contactless power receivers can be powered or charged by being positioned in close proximity to a contactless power transmitter. Such systems have been designed and implemented.
Contemporary contactless power systems are make use of switch-mode inverters, such as the Class, D, DE, E, E−1, F, F−1, EF, EF2, EF3, Phi. The switch-mode inverter converts DC voltage that is provided by a DC voltage source to into a high frequency signal that enables efficient coupling of one or more primary coils to one or more secondary coils. The secondary coils are ultimately connected to one more loads. In the case of a contactless power transfer system the load of an inverter is typically a portable electronic device or some other load device with a variable power requirement. In many instances the load has an input impedance that is variable. The load can use energy or it can be designed to store energy. The load can comprise a voltage regulator and/or a power management system for regulating and relaying the power to an energy consuming or energy storing element. The impedance of the load helps determine the loading condition.
A typical switch-mode inverter comprises an active device, a supply network, and a load network with output terminals for connecting to a load.
The active device is typically a transistor and operates as a switch. The switch alternates between a conductive and non-conductive state. A control signal from a gate drive or clock can be used to operate the switch. The switch is connected to a supply network and a load network. The switching of the active devices helps form an AC signal at the output of the load network.
The supply network relays power from a source the DC voltage source to a terminal of the active device. The DC voltage source can have an output voltage that is variable. The supply network can be a simple inductor and typically comprises passive components. In some cases it may comprise an active device or variable elements for active reconfiguration of the supply network. A reconfiguration of the supply network can be performed depending on the load conditions in order to optimize efficiency or regulate the power which is delivered from the source.
The load network relays power to the load device from a terminal of the active device and supply network. The load network typically comprises passive components. In some cases it may contain an active device or variable for active reconfiguration of the load network. A reconfiguration of the load network may be performed depending on the load conditions in order to optimize efficiency and/or regulate the power delivered to the load.
The load network includes one or more primary coils for inductively coupling to one or more secondary coils. Because of size mismatches and restrictions on the use of bulky core materials, the coupling between the primary and secondary coils can be weak thereby reducing efficiency, power delivery, or both.
In order to compensate for weak coupling between primary and secondary coils, typical inductive charge systems typically operate at frequencies greater than 50 kHz. At these higher operating frequencies soft-switching inverters, such as the Class E, E−1, are preferred because they are more efficient than hard-switching inverters. High efficiency is preferable for environmental and regulatory reasons as well as practical reasons such as minimizing heat dissipation.
Soft-switching describes a mode of operation where an active device, such as a transistor, will switch when either the voltage or current across the transistor is zero. Soft-switching eliminates losses that normally occur with hard switching due to switch capacitance and the overlap of voltage and current in the switch. For example, in the case of zero voltage switching, the voltage across a transistor to swings to zero before the device turns on and current flows. Likewise, at turn-off, the voltage differential across the active device swings to zero before it is driven to a non-conductive state.
A practical system is preferably capable of matching the power supplied to the power demanded by a load device. This is important because many load devices have variable power requirements. If the power delivered does not match power required, the excess energy can be dissipated as heat. A load device can have an input impedance that is variable because of a power requirement that is variable (see FIG. 5 for a graph of resistance versus changing time for a typical cell phone battery). The input impedance of the load device can change by an order of magnitude. The input impedance of a voltage regulator connected to a portable electronic device can change by two orders of magnitude. The variable impedance of a load device makes the implementation of contactless power system difficult.
The following two characteristics of soft-switched inverters found in typical contactless power system make the adaptation to a load device with a variable impedance challenging: 1. Most switch-mode inverters have high efficiency over a narrow range of impedances. As an example, a class E inverter typically operates under, high-efficiency soft-switched conditions over a factor of two in load impedance (see FIG. 3) (Raab, 1978). (see FIG. 2 for a graph of efficiency versus normalized resistance for typical switch-mode inverters); 2. The output power vs. load impedance relationship of a switch-mode inverter is different than the output power vs. load impedance relationship of a DC supply (See FIG. 4 for a graph of power delivery vs. load resistance for a DC supply and an inverter). Because of this, a load device's pre-existing power management control system can fail to appropriately regulate the power delivered to the load device which can lead to component failure.
Due to the above described characteristics a contactless power system is likely to encounter one or more of the following problems: 1) over-voltage and/or under-voltage conditions throughout the circuit; 2) excess or inadequate power delivery to individual loads 3) power oscillations; 4) heat problems; and 5) low efficiency.
Notably, a class D inverter architecture does not share the unfavorable characteristics and resulting problems of the other soft-switched inverters. Class D inverters are optimized for driving an impedance looking into the load network that has zero-phase angle (ZPA), and works for positive phase angles. Zero phase angle operation can be maintained by eliminating the reactance in a circuit of by using a combination of control functionalities, including, but not limited to, frequency, and tank circuit control (see FIGS. 7 and 8). A contactless power system with other soft-switched inverter architectures would be expected to make use of similar control functionality because of their sensitivity to the input impedance of the load(s). (Laouamer, R., et al., “A multi-resonant converter for non-contact charging with electromagnetic coupling,” in Proc. 23rd International Conference on Electronics, Control and Instrumentation, November 1997, Vol. 2, pp. 792-797; Abe, H., et al., “A non-contact charger using a resonant converter with parallel capacitor of the secondary coil,” in Proc. Applied Power Electronics Conference and Exposition, 15-19 Feb. 1998, Vol. 1, pp. 136-141; Joung, G. B. et al., “An energy transmission system for an artificial heart using leakage inductance compensation of transcutaneous transformer,” IEEE Transactions on Power Electronics, Vol. 13, pp. 1013-1022 November 1998; Lu, Y., et al., “Gapped air-cored power converter for intelligent clothing power transfer,” in Proc. 7th International Conference on Power Electronics and Drive Systems, 27-30 Nov. 2007, pp. 1578-1584; Jang, Y., et al., “A contactless electrical energy transmission system for portable-telephone battery chargers,” IEEE Transactions on Industrial Electronics, Vol. 3, pp. 520-527, June 2003; Wang, C., et al., “Power transfer capability and bifurcation phenomena of loosely coupled inductive power transfer system,” IEEE Transactions on Industrial Electronics, Vol. 51, pp. 148-157, February 2004; Wang, C., et al., “Investigating an LCL load resonant inverter for inductive power transfer applications,” IEEE Transactions on Power Electronics, Vol. 19, pp. 995-1002, July 2004; Wang, C., et al., “Design consideration for a contactless electric vehicle battery charger,” IEEE Transactions on Industrial Electronics, Vol. 52, pp. 1308-1314, October 2005) Control functionality adds to the cost and complexity of a system and detracts from the commercial viability.
To enable better control functionality and to ensure proper operation of the system, communication systems between the power supply and the load have been proposed (see FIGS. 6 and 9). Such communication systems also add undesirable cost to the system.
The previously described control functionality has been implemented in both contactless power transmitters and contactless power receivers. Control functionality in the receiver has been considered of particular importance when multiple loads require power from the same transmitter. To support multiple loads, it has been proposed that receiver units incorporate mechanisms such as, but not limited to, variable inductance and duty cycling. These mechanisms allow multiple loads to receive power from the same source by giving load devices a mechanism to protect themselves from over-voltage and/or current conditions (FIG. 6). These mechanisms are of high importance because loads without such mechanisms will continue to receive power even when they no longer require power. The power will be dissipated as heat in the load device. Contemporary batteries will not charge at temperatures over 50° C. These systems also add undesirable cost to the system.